The present invention relates to a process for removing cyclic acetals from wastewater streams over heterogeneous acid catalysts.
Cyclic acetals such as 1,3-dioxane, 1,3-dioxolane and their derivatives are prepared by reacting diols and aldehydes. 1,3-Dioxolane is synthesized, for example, from ethylene glycol and formaldehyde, the reaction being carried out in the presence of acid catalysts. Acid catalysts used here are the most varied Brxc3x8onstedt or Lewis acids, but also cation exchangers, such as sulphonated polystyrene resins (Chopade, React. Funct. Polym. 1997, 34, 37-45). After the main quantity of the desired product has been isolated, the wastewaters from such acetal preparation processes, however, still always contain certain amounts of acetals whose separation from the wastewater poses difficulties. In the preparation of xcex1,xcfx89-diols, such as 1,2-diols, 1,3-diols or 1.4-diols, certain amounts of cyclic acetals can also be formed, due to side reactions of these diols with aldehydes formed by oxidation, and can pass into the wastewater.
Compared with open-chain acetals, the cyclic acetals are most stable. They are stable, especially, to bases and oxidizing or reducing media. Under the conditions in a biological sewage treatment plant (neutral pH and oxidizing media), cyclic acetals are thus not attacked or degraded.
It is already known that organic compounds such as phenol and chlorobenzene can be removed from wastewaters by adsorption on absorber resins and activated carbon (Ullmann, 1995, B8, pp. 126-128). However, such processes have the disadvantage that the adsorber materials used are active for a limited time and must therefore be renewed. When such resins are regenerated with methanol or other solvents, in addition, a further liquid stream to be disposed of is produced.
An object of the present invention is to provide a process by which the content of cyclic acetals in wastewater streams can be reduced.
The invention relates to a process for removing cyclic acetals from wastewater streams which is characterized in that the cyclic acetal-containing wastewater streams are passed over a heterogeneous acid catalyst.
The wastewater streams used in the inventive process preferably comprise 5-, 6- or 7-membered cyclic acetals or their substituted derivatives. These wastewater streams are customarily produced in the synthesis of the cyclic acetals. However, they can also arise, as described above, in the synthesis of the respective xcex1,xcfx89-diols underlying the cyclic acetals, i.e. the 5-membered acetals based on 1,3-dioxolane in ethylene glycol preparation, the 6-membered acetals based on 1,3-dioxane in propylene glycol preparation and the 7-membered acetals in 1,4-butanediol preparation.
Particular preference is given to 1,3-dioxolanes of the general formula (I) 
where
R is identical or different and represents hydrogen, halogen or an unbranched or branched, saturated or unsaturated C1-C7 hydrocarbon radical which can optionally be substituted by one or more halogen radicals,
or 1,3-dioxanes of the general formula (II). 
where
R has the meanings specified for the general formula (I).
The 1,3-dioxolanes of the general formula (I) and the 1,3-dioxanes of the general formula (II) are preferably substituted by one or two radicals R.
The inventive process has particularly utility for wastewater streams which comprise 1,3-dioxolane, 1,3-dioxane, their respective halogen-substituted derivatives or their respective 2-mono-(C1-C5)-alkyl, 2,4-di-(C1-C5)-alkyl derivatives.
In particular, the radical R in the general formulae (I) and (II) takes the meanings fluorine, chlorine, bromine, iodine, methyl, ethyl or chloromethyl.
The concentration of the cyclic acetals in the wastewater stream can be in the range 0.1-100 000 ppm, preferably 20-10 000 ppm, in particular 1-5 000 ppm.
In the wastewater stream, one or more different cyclic acetals of the general formulae (I) and (II) can be present. In addition to the cyclic acetals, the wastewater streams can, if appropriate, also comprise other organic substances or solvents, such as aliphatic hydrocarbons or ethers, which are inert to the heterogeneous acid catalyst under the respective reaction conditions in the inventive process.
The inventive process is customarily carried out in such a manner that the wastewater stream loaded with the cyclic acetal or cyclic acetals is passed over the heterogeneous acid catalyst at a temperature in the range 10-120xc2x0 C., preferably 20-80xc2x0 C., a pressure in the range 0.5-20 bar, preferably 0.5-10 bar, in particular 0.8-2 bar, and an LHSV less than 200 hxe2x88x921, preferably in the range 0.1-25 hxe2x88x921, particularly preferably 0.1-10 hxe2x88x921.
The heterogeneous acid catalysts used in the inventive process are known in principle. Preferably, the heterogeneous acid catalyst is a cation exchanger in the H+ form. Such acid cation exchangers are macroporous or in the form of a gel and are commercially available. They can be prepared, for example, by copolymerization of vinyl monomers and divinyl crosslinkers, if appropriate in the presence of solvents, or by condensation of phenol and formaldehyde. Vinyl monomers which can be used are, for example, styrene or acrylic esters; divinyl crosslinker is, for example, divinylbenzene. Acid groups are, for example, carboxyl group, phosphonic acid groups or sulphonic acid groups.
Preferably, macroporous acid cation exchangers are used. The mean pore radius of such macroporous cation exchangers can be, for example, in the range 50-1200 xc3x85, preferably 70-500 xc3x85. The acid cation exchangers can be used, for example, as polymer beads having particle sizes of 0.1-2 mm or as powder resin having particle sizes of 10-100 xcexcm.
The use of a macroporous or gel-form acid SO3H-group-containing cation exchanger having a degree of crosslinking of 2-65% and a surface area per unit volume of 5-750 m2/g of dry cation exchanger is particularly advantageous.
In the inventive process, a chemical degradation of the cyclic acetals takes place over the heterogeneous acid catalyst, in that they are cleaved into their starting components, i.e. diols and aldehydes. These can then be successfully degraded in the realm of biological wastewater treatment. As a result, further chemical work-up of the wastewater after the inventive process is unnecessary.
In addition, the heterogeneous acid catalysts used do not lose activity in the course of the inventive process, as do the adsorber materials known from the prior art for separating off phenols and chlorobenzenes, but have a considerably longer service life, since the cleavage products of the cyclic acetals are discharged again with the wastewater and do not remain on the heterogeneous catalyst. This makes a critical contribution to the economic attractiveness of the inventive process.